Nano materials refer to substances at least one dimension of which is within a nano scale range (1-100 nm) in a three-dimensional space, and such materials typically exhibit different melting point, magnetic property, optical property, thermal conductivity and electrical conductivity from bulk materials due to nano-size effect, therefore, they have extensive application prospect in such fields as photoelectrical material, ceramic material, sensor, semiconductor material, catalytic material and medical care. However, nano materials are usually prepared under rigorous conditions and at high cost, so use of these nano materials as additive in preparation for a nano composite material becomes an effective measure to lower the cost and popularize the nano materials. Polyamide, a type of important engineering plastic, has excellent comprehensive properties, including mechanical property, heat resistance, abrasion resistance, chemical resistance and self-lubricating property, and is low in coefficient of friction and easy to process. There are various types of polyamide, like nylon 6, nylon 4, nylon 12 and nylon 6/12; with a large amount of polar amide bonds therein, these materials are extremely suitable for use as a matrix material and other inorganic materials in preparation for composite material, especially for use as a matrix for nano composite material. Researchers have shown that addition of nano particles into polyamide typically imparts to polyamide properties that are not originally present in polyamide, e.g. reinforcement, toughening, abrasion resistance, high temperature resistance and improvement of processability and functionality. For example, compounding of magnetic nano particles and polyamide results in a magnetic nano particle/polyamide composite material that is small in relatively density and can be easily manufactured into a product with high dimensional precision and complex shape, thus overcoming the defect that a product with complex and fine shape cannot be manufactured by original ferrite magnets, rare earth magnets and Al—Ni—Co alloy magnets due to their high hardness and brittleness and poor processability.
At present, common preparation methods for a nano particle/polyamide composite material includes a blending method, a sol-gel method and an in-situ synthesis method.
The blending method refers to mixing of nano particles and polyamide polymer by means of solvent blending, emulsion blending, melt blending and mechanical blending and the like. It has the advantages of good simplicity, convenience, and low cost. Synthesis of nano materials and material are carried out step by step so that the morphology and size of the nano particles can be controlled, however, owing to small size of these nano particles and high viscosity of polyamide, they are difficult to be blended in and uniformly dispersed, which typically will degrade the mechanical properties of the nano particle/polyamide composite material significantly. In order to improve the compatibility between the nano particles and the polyamide composite material, surface modification is normally employed to promote uniform dispersion of the nano materials and strength an action force at an inorganic/organic interface. The procedure of surface modification is not only time-consuming and labor-consuming, but is also quite difficult to reach ideal effects.
As a common method for nano material synthesis, the sol-gel method typically means that a precursor for nano material synthesis is dissolved in a certain solvent and then hydrolyzed or alcoholized to form sol, the sol is then subjected to solvent volatilization or heating to achieve gelation so as to generate nano particles. When the sol-gel method is used for preparing a nano particle/polyamide composite material, a precursor for nano material synthesis is introduced to a polyamide matrix material at first and then hydrolyzed or and gelated in the polyamide matrix to directly generate the uniformly-dispersed nano particle/polyamide composite material. This method has such a characteristic that it can be carried out under mild reaction conditions. Its two-phase dispersion is more uniform than the blending method. But it has the following shortcoming that, in the process of gel drying, volatilization of solvent, small molecules and water is likely to cause shrinkage and embrittlement of the material, furthermore, it is quite difficult to introduce a large amount of the nano particle precursors to the polymer matrix, therefore, the properties of the material are improved to a limited extent.
The in-situ polymerization method means that, nano particles are directly dispersed in monomer for polyamide synthesis and then monomer polymerization is initiated under particular conditions to form a nano particle/polyamide composite material. As an effective measure for synthesizing the nano particle/polyamide composite material, the method has the advantages of completely independent controllability of nano particle filler and wide selection range for polymer matrix. At present, many documents that describe use of the in-situ polymerization method for preparing the nano particle/polyamide composite material have been reported, e.g. LIU, Andong, et al. nano Na-montmorillonite is uniformly dispersed in nylon 6 matrix caprolactam and a montmorillonite/nylon 6 composite material is prepared by anionic ring opening polymerization [For more details, please see: Liu A., Xie T., Yang G. “Synthesis of exfoliated monomer casting polyamide 6/Na+-montmorillonite nanocomposites by anionic ring opening polymerization.” Macromol. Chem. Phys., 2006(207):701-707]. However, the in-situ polymerization method is still difficult to achieve ideal dispersion of the nano particles in the polyamide matrix, and since the nano particles serving as raw material are usually high in production cost and poor in stability, high difficulty is brought to industrial production of the nano particle/polyamide composite material.
Lactam shows strong polarity because amide bonds are contained in a ring structure, as a result, and in addition to use as a solvent for dispersing inorganic nano materials excellently (e.g. montmorillonite, nano-silica, nano-hydroxyapatite), it has a quite strong dissolving capacity for a plurality of water-soluble inorganic salts (e.g. AgNO3, ZnCl2, FeCl3 and NaOH) and oil-soluble metal or semimetal organic compounds (e.g. carbonyl iron and ethyl orthosilicate). Therefore, the nano material can be synthesized by dissolving the water-soluble inorganic salts or the oil-soluble metal or semimetal organic compounds in the lactam solvent and then adopting a proper method. For example, Gao, et al, have synthesized superparamagnetic ferroferric oxide with a particle diameter less than 20 nm respectively by carbonyl iron and ferric trichloride in a butyrolactam solvent [For more details, please see the document below: One-Pot Reaction to Synthesize Water-Soluble Magnetite Nanocrystals, Chem. Mater., Vol. 16, No. 8, 2004; Preparation of Water-Soluble Magnetite Nanocrystals from Hydrated Ferric Salts in 2-Pyrrolidone: Mechanism Leading to Fe3O4, Angew. Chem. Int. Ed. 2005, 44, 123-126]. So far, most of the documents or patents have been reported only in the aspect of use of lactam as a solvent for nano material synthesis, in addition, in order to obtain pure nano material powder, complex post-treatment procedures are needed in general, such as washing, separation and drying, therefore, the cost of nano material synthesis is remarkably increased and the problems of environmental pollution and increase of energy consumption are aroused. If the nano particle/lactam mixture obtained in the nano material synthesis process featured by use of the lactam as a solvent is not separated, instead, the lactam is directly polymerized to form polyamide polymer, this will be an effective measure for preparing the nano particle/polyamide composite material.